Motor neuron disorders include amyotrophic lateral sclerosis, autosomal recessive spastic paraplegia, hereditary spastic paraplegia, primary lateral sclerosis, progressive pseudobulbar palsy, progressive muscular atrophy, progressive bulbar palsy, and postpolio syndrome. Symptoms characteristic for a specific type of motor neuron disorder vary according to the part of the nervous system most affected. Median age of onset for developing a motor neuron disorder is 55 years, with a higher incidence in males. 5% of motor neuron cases are familial with autosomal dominant inheritance.
Clinical aspects of motor neuron disorders involve debilitating symptoms resulting commonly in death. Amyotrophic lateral sclerosis, for example, involves muscular weakness, atrophy and signs of anterior horn cell dysfunction. The site of onset is random, and progression is asymmetric. Cramps are common and may precede weakness. Visible muscle twitches (fasciculations), spasticity, hyperactive deep tendon reflexes, extensor plantar reflexes, and signs of corticospinal tract involvement soon follow. Dysarthria and dysphagia are due to involvement of brain stem nuclei and pathways. Sensory systems, voluntary eye movements, and urinary sphincters are spared. Rarely, a patient survives 30 years, while 50% die within three years of onset, 20% live for 5 years, and 10% live 10 years.
Autosomal hereditary spastic paraplegia, for example, involves an age of onset varying from childhood to old age. Pathologic findings include degeneration of the descending corticospinal tracts and fasciculus gracilis, despite the absence of sensory findings. Anterior horn cell dropout has been reported. The disorder may occur with other neurologic abnormalities, including spinocerebellar and ocular symptoms (Ferguson-Critchley syndrome), extrapyramidal symptoms, optic atrophy, retinal degeneration (Kjellin syndrome), mental retardation or dementia, and polyneuropathy. The etiology of these syndromes is unknown. Symptoms and signs include progressive gait difficulties, hyperreflexia, clonus, and Babinski's sign. Sensory and sphincteric functions are usually spared. Arms may also be affected.
Motor neuron disorder diagnostic features include onset during middle or late adult life and progressive, generalized motor involvement without sensory abnormalities. Nerve conduction velocities are normal until late in the disease. Electromyography is the most useful test, showing fibrillations, positive waves, fasciculations, and giant motor units, even in unaffected limbs.
There is no specific treatment for motor neuron disorders. Physical therapy may help maintain muscle function. Patients with pharyngeal weakness are fed with extreme care and require a gastrostomy. Baclofen is used to reduce spasticity. Quinine or phenytoin is used to decrease cramps. A strongly anticholinergic drug, such as amitriptyline, is used to decrease saliva production. Surgery to improve swallowing has limited success in patients with progressive bulbar palsy.
Improved therapies aimed at decreasing motor neuron disorder symptoms are needed. Preventive strategies aimed at preventing the onset of motor neuron disorders are also needed. In addition, improved methods of identifying individuals at risk for developing motor neuron disorders are needed.